This invention relates to apparatus for the mass storage of data and more particularly to a method and means for reading data stored on a record.
The volume production of recordings of television programs on 12 inch discs, with play back times of up to one-half hour has been demonstrated. Assuming about 1 million bits of information per frame, 30 frames per second, this means that 10.sup.10 bytes are being stored on a single 12 inch disc. The error rate that can be tolerated on television displays is fairly high as the frames are being rapidly changed and the eye integrates out many of the errors that may occur, particularly as it is unlikely that an error would occur in the same place on consecutive frames. An error, in say, 10 identifiable locations, on a frame, or one in 10.sup.5 bits could be comfortably tolerated, thus easing the requirements in perfection of the recording technique and the playback head, and the necessity for keeping the record free from dust and scratches.
If the same technique were to be used for the storage of computer generated binary coded information, the error rate would have to be much lower. For example, if such a device were to store information for a telephone directory, the change in a single digit would give a wrong number, thus an error rate of one in 10.sup.10 would be more appropriate. However, books, which consist of straight reportage, can use the television recording techniques since, a misspelled word, while it may be a source of amusement, rarely, in context, represents any loss of information. By using an 8-bit ACSII code, large amounts of literature could be condensed on a single record. At 10.sup.10 characters, assuming an average of 5 characters per word, 2000 words per page and a 500 page volume, on a two-sided record, this amounts to 4000 volumes per disc; more novels than most people read in a lifetime.
While a fairly high cost can be borne in the capital equipment for making such discs, both the cost of the disc and the capital equipment cost of the readout device must be reasonably low in order to appeal to a mass market.